The present invention relates to a method of recovering metal from ore by bio-leaching. In a first aspect, the present invention relates to a method of recovering metals such as nickel, cobalt, zinc, and copper from ore by a novel bio-leaching method. In another aspect, the present invention also includes the recovery of precious and/or platinum group metals from ore that has been subjected to a bio-leaching process according to the first aspect of the invention.
Valuable base metals such as nickel, cobalt, zinc, copper and the like are distributed throughout ores. In general, one or more of these base metals may be present in the ores, which can be in the oxide, sulfide, mixed, and other forms. In addition, it is often found that precious and/or platinum group metals are associated with the above base metals. It would therefore be desirable to recover not only the base metals but also the precious and/or platinum group metals using a simple, economical process.
There are many known processes to recover base metals. Many of them require high capital costs and are environmentally unfriendly. For example, one method of recovering these base metals is by hydrometallurigical processing using sulfuric acid to leach the metals from the ore. The problem with this type of process is that it requires the construction and operation of a sulfuric acid plant, which is expensive.
Another problem with recovering these types of metals is exemplified by the problems associated with the recovery of zinc from mixed ores (those ores in which the ore is in the oxide and sulfide form). Using sulfuric acid to recover zinc requires a very large amount of acid to effectively leach the zinc oxide. In addition, the leaching of zinc sulfide generally requires leaching under pressure.
An alternative method to the recovery of these metals is by a flotation process. It is known, however, that the flotation process is generally not effective when the ore is a mixed ore.
Another process is described in Canadian patent application No. 2,065,491. In this application, nickel in nickel-containing ores may be recovered by a bio-leaching process. The described bio-leach process can be carried out by heap leaching where the bio-leach solution trickles gravimetrically through the heap.
Another similar process is described in U.S. Pat. No. 5,626,648. This patent teaches a bioleaching method for recovering nickel from lateritic ore and nickel-containing sulfidic material. Where the ore is a nickel containing lateritic ore, this patent teaches the use of microorganisms that systemically produce an organic acid conducive to forming a complex with nickel. The preferred organic acids are stated to be oxalic, pyruvic, citric, tartaric, malonic, and other acids. Where the ore is a nickel-containing sulfidic material, this patent teaches the use of a biooxidizing bacterium that is selective to the leaching of the sulfidic ore.
While the above processes are satisfactory, one problem with them is that they are specific to the particular type of ore to be leached. The present invention addresses that problem by providing an economical and effective process for recovering base metals from mixed ores, as well as oxide and sulfide ores, without regard to the ore content. In addition, the above processes do not contemplate the further recovery of precious and/or platinum group metals.
The present invention provides a process for economically recovering base metals from ore. In general, the base metals are selected from the group consisting of nickel, cobalt, zinc, and copper. The present invention also provides a process for recovering precious and/or platinum group metals. The precious group metals are selected from the group consisting of silver and gold. The platinum group metals are selected from the group consisting of rhenium, osmium, iridium, platinum, technetium, ruthenium, rhodium, palladium, and mixtures thereof.
In general terms, the process includes the steps of contacting a metal-containing ore with an effective amount of at least one microorganism that is selective to metabolize sulfur (sulfur selective microorganism) to systemically produce sulfuric acid in amounts effective to leach base metals from the ore to form a base metal-rich leachate and an ore residue. A sulfur-containing compound is mixed with the microorganism before, during, or after the microorganism contacts the ore. The addition of the sulfur-containing compound allows the amount of systemically produced sulfuric acid to be adjusted depending on the particular type of ore to be leached.
The base metal-rich leachate is separated from the ore residue. The base metal-rich leachate may be further processed to recover the base metals. In addition, the ore residue may be subjected to a flotation process to concentrate the precious and/or platinum group metals. The concentrate can be upgraded in a known manner to produce high-grade precious and/or platinum group metal(s).
The metal containing ore may provide the base metal in an oxide form, a sulfide form, and/or a mixed form (i.e., a form where the ore contains both oxide and sulfide forms). In other words, the ore may contain one or more of nickel, cobalt, zinc, and copper with each present in the ore in substantially all oxide form, all sulfide form, or a mixture of both oxide and sulfide forms. The metal containing ore may also contain one or more precious and/or platinum group metals.
The sulfur-containing compound may be any compound suitable to provide a source of sulfur. Therefore, the sulfur-containing compound can include, but is not limited to elemental sulfur, iron sulfide, nickel sulfide, cobalt sulfide, zinc sulfide, and copper sulfide.
Microorganisms suitable to metabolize the sulfur include those that oxidize the sulfur to provide sulfuric acid. The oxidizing microorganism metabolizes (oxidizes) the sulfur present in the ore and/or sulfur-containing compound to systemically produce sulfuric acid that, in turn, will leach the metal from the metal-containing ore. The useful microorganisms are distinguished from those described in U.S. Pat. No. 5,626,648, which produce organic acidsxe2x80x94not sulfuric acid. According to one feature of the present invention, the systemically produced sulfuric acid is the only sulfuric acid used for leaching the ore and no additional sulfuric acid is required or needed.
One aspect of the present invention includes a process for bio-leaching base metal-containing ore to recover the metal in a useable form. In one embodiment, the process of the invention involves the heap, vat, or agitation bio-leaching of metal-containing ore by contacting the base metal-containing ore with an effective amount of at least one microorganism that is selective to metabolize sulfur and mixing a sulfur-containing compound with the microorganism before, during, or after the microorganism contacts the ore to systemically produce sulfuric acid in amounts effective to leach the base metal from the ore.
In a preferred aspect of the present invention, the metal-containing ore is formed into a heap having a top and a bottom. The sulfur selective microorganism is placed into intimate contact with a substantial portion of the metal-containing ore. Preferably, a solution containing the sulfur selective microorganism is caused over a period of time to trickle gravimetrically from the top to the bottom through the interstices of the heap.
Desirably, a sulfur-containing compound is mixed with the sulfur selective microorganism before applying the solution to the heap, mixed with the microorganism during the application of the microorganism to the heap, or mixed with the ore prior to applying the microorganism to the heap. More preferably, the sulfur-containing compound is mixed with the sulfur selective microorganism and any other ingredients to form a solution. It is to be understood that the sulfur-containing compound can be added at any time during the process so long as at some point in time there is intimate contact with the sulfur selective microorganism so that the microorganism can oxidize the sulfur to systemically form sulfuric acid.
The ore residue remaining from the above-described leach process can then be further processed to provide precious and/or platinum group metals in a form suitable for commercial sale. For example, the ore residue may be separated from the heap and ground to a size suitable for flotation processing. The ground ore residue is then subjected to mineral flotation collectors to concentrate the precious and/or platinum group metals. The concentrate may then be upgraded to produce a suitably high grade of precious and/or platinum group metal(s) for sale or refining.
In one embodiment of the present invention, the process includes heap leaching according to the methods described above and separating the solution from the residue. The solution is treated with a microorganism to provide a sulfide concentrate simultaneously, the residue is treated in a rougher flotation process to provide a rougher concentrate and tails. The sulfide concentrate and rougher concentrate are combined in a cleaner flotation process to provide a final concentrate from which base and/or precious metals may be recovered.
A benefit of the present invention is that the size of the sulfuric acid plant can be greatly reduced. As a result, the capital and operating costs of the methods of the present invention are substantially less than if a sulfuric acid plant were needed.
As used in the specification and appended claims, the term microorganism includes both the singular and the plural. In addition, all percentages used in the specification and appended claims are by weight, unless specifically noted otherwise.